Rapid adsorptive removal of cationic and anionic dyes from aqueous solution by a Ce(III)-doped Zr-based metal–organic framework

Porous metal–organic frameworks (MOFs) exhibit excellent characteristics for the adsorptive removal of environmental pollutants. Herein, the surface charge, adsorption sites, and adsorption spaces in the pore channels of UiO-67 nanocrystals were modulated by Ce(III) doping to reveal the effects of these parameters on the removal of organic dyes such as methylene blue (MB), methyl orange, and Congo red from aqueous solution. The MB adsorption capacity of Ce(III)-doped UiO-67 (398.9 mg g−1) exceeded that reported for pristine UiO-67 by 85%, which was rationalized by zeta potential measurements and adsorption isotherm analyses. As a result, it was revealed that whereas electrostatic repulsion occurred between MB and pristine UiO-67, electrostatic attraction occurred between MB and Ce(III)-doped UiO-67. Furthermore, Ce(III) doping promoted π–π interactions between the adsorbent and the adsorbate. Thus, Ce(III) doping increased MB adsorption capacity despite decreasing the number of adsorption sites and spaces in channels. The obtained results shed light on the mechanism of organic dye adsorption on MOFs and revealed that the synergetic interplay of electrostatic and π–π interactions, adsorption sites, and adsorption spaces increases the adsorption capacity for cationic dyes while decreasing that for anionic dyes.